Systems with an SCR catalytic converter (selective catalytic reduction) are well suited for removing NOx emissions, specifically in the case of exhaust gases of diesel engines. In the case of an active SCR system, ammonia (NH3) is dosed in the SCR catalytic converter where it is adsorbed on the catalytic converter and reacts with NO and NO2 from the exhaust gas. NH3 is typically dosed not directly but mostly in the form of a urea solution that is partially converted into NH3 after the injection. In the case of a passive SCR system, there is no active injection of NH3 or urea upstream of the catalytic converter. Instead of this, NH3 is produced by another component upstream of the catalytic converter, such as for example a LNT (lean NOx trap), once the engine is operating in the rich mode.
If not enough NH3 is being dosed or stored in the catalytic converter, or if the temperature is unsuitable for complete NOx conversion, not all the NOx is converted, and there will be a NOx slip through the catalytic converter. If, however, too much NH3 has been dosed and stored in the catalytic converter, a desorption can occur. An NH3 desorption usually occurs after a rapid temperature rise, for example, as a consequence of a rising engine load. However, it can also occur given overdosing of NH3 at a constant temperature.
Information relating to the concentration of NOx and NH3 downstream or behind the catalytic converter is of interest for the purpose of accurately monitoring the processes in the SCR catalytic converter. There are two types of sensors, NH3 sensors and NOx sensors, for this purpose. Whereas NH3 sensors measure only NH3 concentrations or quantities, NOx sensors are sensitive both to NOx and to NH3. This leads to difficulties in measurement inasmuch as it can be difficult to establish whether the sensor is measuring just NOx values or NH3 values.
Thus, in one embodiment, a filtering method for a NOx sensor of an exhaust gas system having an SCR catalytic converter comprises determining NOx concentration upstream of the catalytic converter, measuring NOx concentration downstream of the catalytic converter, modeling NOx conversion, modeling NH3 slip behavior, calculating a NOx error of the NOx modeling, calculating an NH3 error of the NH3 modeling, assigning data of the NOx sensor as NH3 measured values when a ratio of the NOx error to the NH3 error is greater than an upper threshold value, and assigning data of the NOx sensor as NOx measured values when the ratio of the NOx error to the NH3 error is smaller than a lower threshold value.
With the aid of this filtering method, it is possible, for example, to establish or fix, for subsequent data processing by a controller of the exhaust gas system, whether the signal measured by the NOx sensor is a NOx measured value or an NH3 measured value. Two different models are created and/or adapted continuously. One model describes the NOx conversion in the catalytic converter, and the second model describes the behavior of the NH3 slip through the catalytic converter. The actual NH3 release process is complex and difficult to describe through a simple model, but it is possible, nevertheless, to use the two models described to establish the measurement mode of the NOx sensor without an additional NH3 sensor. Finally, the prediction quality or the error of the two models can be calculated and compared. In the case of a NOx slip, both models typically display similar levels of accuracy, while the accuracy of the NOx model is substantially degraded given an NH3 slip. An NH3 model based on a linear-time algorithm can also predict a NOx signal in a restricted time window.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.